Welcome to Gaia! ::

nobody was going to ask. o:

The fact alone that this is his issue shows it's a relationship that started based on lust as a primary factor. Many of those are doomed to fail. In order to last love should come before lust. Then if lust dwindles which can happen for instance when children come along a relationship can pull through.

Also most people today are too sex obsessed to begin with, this will lead to relationships failing as well. It is dickish and we don't know how thin she was before and if there are other factors involved. If he wants to help her, encourage her to get healthy and once she is healthy accept the weight that she will have when being healthy naturally.